feat: add extension point for excerpt generation

[guqing]

What type of PR is this?

/kind feature
/area core
/milestone 2.18.x

What this PR does / why we need it:

新增文章摘要生成扩展点用于扩展自动生成摘要的方式

Does this PR introduce a user-facing change?

新增文章摘要生成扩展点用于扩展自动生成摘要的方式

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 16 lines in your changes missing coverage. Please review.

Project coverage is 58.36%. Comparing base (bc10336) to head (2dbf65b).
Report is 97 commits behind head on main.

Files	Patch %	Lines
...in/java/run/halo/app/content/ExcerptGenerator.java	0.00%	6 Missing ⚠️
.../app/core/extension/reconciler/PostReconciler.java	87.17%	5 Missing ⚠️
...ore/extension/reconciler/SinglePageReconciler.java	81.48%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6348      +/-   ##
============================================
+ Coverage     54.51%   58.36%   +3.85%     
- Complexity     3523     3770     +247     
============================================
  Files           646      647       +1     
  Lines         21862    22038     +176     
  Branches       1528     1548      +20     
============================================
+ Hits          11917    12862     +945     
+ Misses         9328     8554     -774     
- Partials        617      622       +5     

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[ruibaby]

/lgtm

需要在 https://github.com/halo-dev/docs 补充文档。

[JohnNiang]

/lgtm
/ping @LIlGG

[LIlGG]

/lgtm

[JohnNiang]

/approve

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JohnNiang, LIlGG

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